The present invention relates to liquid crystal projection system and, more particularly, to a transmission-type liquid crystal projection display using an L-shaped dichroic prism set.
Transmission-type liquid crystal projection systems have advantages of high image quality, small volume, and portability, and can be used for fabricating large-area displays. However, because of different designs of their optical systems, the difficulties in fabrication differ. The operational principle of a liquid crystal display is to modulate the polarization state of light to change the intensity of output light so that gray scales of different brightness can be exhibited. Therefore, polarizing plates are arranged in front of and behind the liquid crystal display to adjust the intensity of light.
Prior art transmission-type liquid crystal projection displays can be divided into single-plate type and triple-plate type according to the count of adopted liquid crystal displays. The three-plate type liquid crystal projection display is more complex, but has better capability in brightness and resolution than the single-plate one. Therefore, high-quality liquid crystal projection displays are mainly the triple-plate type ones until now. A general triple-plate type liquid crystal projection display uses a cross dichroic prism set to synthesize polarized color lights of the three primary wavelengths into one (e.g., the projection displays disclosed in US. Pat. Nos. 5,073,013 and 5,075,798). The cost is higher, and the adopted cross dichroic prism set has problems of higher complexity and higher difficulty in structure and fabrication.
The present invention aims to propose a transmission-type liquid crystal projection display to resolve the above problems in the prior art.
The primary object of the present invention is to provide a transmission-type liquid crystal projection display having an L-shaped dichroic polarizing prism set so as to achieve the effects of high efficiency and low cost.
Another object of the present invention is to simplify the difficulty of a dichroic polarizing prism set used in a transmission-type liquid crystal projection display and the difficulty of fabricating the dichroic polarizing prism set.
According to the present invention, a transmission-type liquid crystal projection display comprises an incident light source. Two dichroic polarizing plates and three reflecting mirrors are used to separate the incident polarized light into three color lights of the three primary wavelengths. Each of the three color lights respectively pass a liquid crystal display and then is projected onto an L-shaped dichroic polarizing prism set, which comprises two dichroic polarizing prisms and an equivalent optical path glass block. Thereby, the color lights of the three primary wavelengths can be synthesized into a single light beam, which then passes a projection lens to be projected onto a screen.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawings, in which: